1. Field of the Invention
The present invention relates to a synchronous digital hierarchy (SDH) transmission system, more particularly to an apparatus for monitoring through-paths in such a system.
2. Description of the Related Art
As will be explained in detail later with reference to the drawings, in a conventional SDH transmission system, the path overhead POH can only be detected by the SDH equipment terminating the path. When an error occurs during transmission over a certain path, however, it is not possible to judge at what stage (at what end office) the error occurred and consequently time is taken for ascertaining the source of the error and recovering the same. Accordingly, in recent years, even the CCITT has established a new protocol so as to enable monitoring of the control bytes or control bits making up a path overhead during transmission, but no equipment satisfying that protocol has yet been developed.